1. Field of the Invention
The present invention relates to an optical position measuring apparatus of the type in which the position or displacement of an object is measured by projecting a radiation beam on the object and detecting the position of the reflected beam spot from the object surface.
2. Related Background Art
The above-mentioned type of measuring apparatus using a radiation beam such as a laser beam is known in the art. Typical examples of the prior art optical measuring apparatus are shown in FIGS. 1 and 2. As seen from the figures, the apparatus generally comprises a light projection optical system for projecting a radiation beam onto an object, a light-receiving optical system for receiving the reflected beam from the object, and a one-dimensional optical sensor. The light-receiving optical system is disposed obliquely to the projection optical axis and functions to form a spot image of the reflected beam on the sensor. The position of the formed image is detected to obtain the necessary information with respect to the measured object.
For example, the change of the position of the spot image on the one-dimensional sensor is detected to measure the displacement of the object in the direction along the projection optical axis.
The above-shown position measuring apparatus using radiation beam such as laser beam is a kind of non-contact measuring apparatus which has many advantages over the conventional contact type position measuring apparatus. First of all, even for such an object made of very soft plastics, measurement can be made without any danger of the object being damaged Secondly, it is very easy to use and especially useful as a distance detector for automation equipments and devices such as a factory work robot.
However, the prior art optical measuring apparatus as described above involves the following problems which cause the apparatus to have insufficient accuracy of measurement and to have only limited applications.
Referring to FIG. 1, the prior art apparatus measures the distance to an object on the principle of triangulation. A laser light source 1 emits a laser beam toward the object the position of which varies in the range of from +a to -a in the direction of the projection optical axis A. The laser beam is projected on the object through a projection optical system 2. The reflected light of the beam spot from the object enters a light-receiving optical system 3. The incident angle of the reflected light to the optical system 3 varies depending on the position of the object. The change of the incident angle is detected as the change of the position of the beam spot on a linear (one-dimensional) optical sensor 4 disposed perpendicularly to the optical axis B of the light-receiving optical system 3.
In the above-shown prior art apparatus, therefore, the projection optical axisA of the projection optical system 2 is oblique to the optical axis B of the light-receiving optical system 3 and the light-receiving surface of the sensor 4 is normal to the optical axis B. This means that the beam on the object and the image of the beam (+a'.about.-a') on the light-receiving surface are not in conjugated relation to each other. As a result, the image of the beam spot formed is blurred. This prior art measuring apparatus measures the distance by electrically detecting the center of the beam. Consequently, the range of measurement allowable for the apparatus is limited to the range within which the blurred image can be processed electrically.
Another problem is related to the optical sensor 4. As the sensor 4 in the apparatus as shown in FIG. 1, there may be used a linear image sensor or a position sensitive device (it will hereinafter be referred to as PSD in brief). But, these sensors now available have their own limitations of performance. Because of this, the resolving power for the detection of the position of the light beam image on the sensor is inevitably limited however highly the image may be processed electrically. In order to improve the accuracy of measurement, therefore, it is wished to enlarge the movement of the beam on the one-dimensional optical sensor relative to the displacement of the object. In other words, it is desirable to increase the magnification power of the light-receiving optical system 3 as much as possible. However, in this case, if a magnifying system is used as the light-receiving optical system 3, the depth of focus on the object side will be rendered so small that only a very small measurable range may be obtained even when electrical processing is carried out well. To obtain a larger measurable range it is necessary to use the light-receiving optical system 3 as a minifying optical system. In this case, however, it is impossible to improve the accuracy of measurement.
The position sensitive device (PSD) generally has a structure comprising an n-type substrate and a p-type substrate uniformly laid on the n-type substrate. A pair of electrodes for reading out signals are provided at both ends of the p-type substrate and a reference electrode is provided at the n-type substrate. The electric current flowing through the pair of electrodes varies depending upon the position of the spot light on the surface of the p-type substrate.
A further problem is caused by the fact that the optical axis B of the light-receiving optical system 3 is oblique to the projection optical axis A. Due to this fact, the magnification rate of the distance between two points on the sensor 4 (for example, the distance from the center 0' to +a' or to -a') corresponding to the distance between arbitrary two points on the optical axis A (for example, the distance from the center 0 to +a or -a) is not constant but variable. To compensate for this a correction is needed. Furthermore, the sensitivity for detection varies according to the position of the measured point (the magnification rate gradually decreases and the sensitivity gradually lowers in the direction in which the measured point is more distant from the light-receiving optical system 3).
The second prior art apparatus shown in FIG. is an optical position measuring apparatus which has been proposed to overcome one of the above-mentioned problems, that is, the problem of the blurred image formed on the light-receiving surface of the sensor 4.
As seen from FIG. 2, in this apparatus, the principal plane PP of the light-receiving optical system 3, the light-receiving surface of the sensor 4 and the optical axis A of the projection optical system 2 are so disposed as to intersect each other at a point P. In this prior art apparatus, therefore, the point (+a.about.-a) on the projection optical axis A and the image (+a'.about.-a') on the light-receiving surface are in conjugated relation to each other. Consequently, there is no problem of the blurred image. However, even this prior art apparatus involves again the problem that the magnification rate is variable according to the position of the measured point. Also, the accuracy of measurement attainable by the apparatus is still unsatisfactory.